This invention relates to a rotation preventing mechanism for preventing an orbiting member from being rotated relative to a fixed member, and to a scroll-type fluid displacement apparatus comprising the rotation preventing mechanism.
For example, Japanese Unexamined Patent Publications (JP-A) Nos. 33811/1993 and 87131/1993 disclose scroll-type compressors or fluid displacement apparatus with rotation preventing mechanisms. Each of the scroll-type compressors has a fixed member, an orbiting member carrying out orbital motion or revolution with respect to the fixed member, a pair of races attached to the fixed member and the orbiting member, respectively, to face each other, and a ball interposed between the races. More specifically, each of the races has a rolling groove surface which has an annular shape corresponding to an orbit of the orbital motion. The ball is received by the rolling groove surface to inhibit rotation of the orbiting member during the orbital motion. The rolling groove surface has a radius of curvature substantially equal to a radius of the ball in a cross-section thereof.
In the above-mentioned rotation preventing mechanism, one of the races which is attached to the orbiting member revolves or orbits along the orbit of the orbital motion to have an orbiting radius determined by the rolling groove surface. In a design stage, it is supposed that the ball rolls along the bottom of the rolling groove surface.
In practical manufacture, however, dimensional tolerance of parts and dislocation In assembling are inevitable. This results in a dimensional difference between the radius of the rolling groove surface and the actual orbiting radius of the above-mentioned one race. Therefore, the ball may be shifted from the bottom of the rolling groove surface to climb up an outer or an inner peripheral portion of the rolling groove surface. In this event, the races are subjected to thrust force from the ball along different lines of action.
In addition, since the climbing level of the ball fluctuates, a supporting portion which actually supports the orbiting member or a movable scroll member of the scroll-type compressor may be worn or deformed due to the thrust force. As a result, the rotation preventing mechanism is shortened in its lifetime.
Furthermore, such fluctuation in climbing level of the ball results in unreliable support of the orbiting member and presence of a gap between the orbiting member and the fixed member. Thus, compression efficiency of the scroll-type compressor is deteriorated.